Signal inducing devices, sometimes referred to as speed sensor rings are frequently used for a plethora of technical devices. As an example, they are used to determine the speed, and sometimes even a position, of a device that is turning with respect to a (main) part of a machine being at rest.
An example for this is a turning axle of a vehicle (where wheels are attached to) or a crankshaft of a combustion engine.
Due to the wide and frequent use of such speed sensor rings, a huge variety of speed sensor rings is known in the state of the art which all work reasonably well.
Sometimes, however, problems arise in some special technical applications that are not known in the context of other technical applications (although they sometimes seem to be closely related with each other).
As an example, toothed wheels made of a soft magnetic material are frequently used as a signal inducing device for a magnetic sensor, where the combination of the toothed wheel and the sensor are used for determining the turning speed of an axle that is connected to a wheel. Such an arrangement is used for determining the speed of a vehicle and—as a more elaborate example—as a sensor arrangement for an anti-lock system for breaking (which relates to speed information as well). While such an arrangement is working well for a lot of applications, problems can occur, if the device is used for hydraulic systems. Here, the presence of oil near the signal inducing device is almost unavoidable. Sometimes, the toothed wheel has to be arranged in an area of the vehicle, where the toothed wheel is partially or even fully immersed in hydraulic oil. Now, the problem arises that the teeth and the gaps between the teeth produce a hydraulic friction in the oil bath. In particular at higher turning speeds of the axle/toothed wheel, a significant loss of mechanical energy (and consequently an appropriately high generation of thermal energy) will occur when using such a toothed wheel. Of course, this adverse behaviour is to be avoided.
For such applications, an alternative solution has been suggested, where an elastic band is used as a base for a signal inducing device. At regular intervals, a magnetic coating is arranged of the base material. Afterwards, a thin coating is applied to the outer surface of the device to make the surface as smooth as possible. A disadvantage of this solution is that still some roughness exists on the outside. Furthermore, the device is quite cost intensive to produce. Yet another disadvantage is the use of magnetic material (hard magnetic material to be exact). When the material is magnetised, it is prone to agglomeration of soft or hard magnetic materials. However, such materials can be around in form of small metallic particles in the hydraulic oil, where the small metallic particles can be present due to wear of metallic components. This can even lead to a destruction of the sensor. It is easy to understand that this is not what one is looking for.